Question

i.How many grams of neon (Ne) will have the same number of a $4g$ calcium (Ca)? Atomic masses : Ne= $20{\text{ }}u$ and Ca = $40{\text{ u}}$
ii.Calculate the molar mass of $S_8$. (atomic mass of S $= 32{\text{ }}u$)

Answer 1

Read the question properly to understand it. Number of atoms in any element is given by $Na{\text{ }}\left( {avogadro's{\text{ }}number} \right){\text{ }} \times {\text{ }}\dfrac{{mass}}{{atomic{\text{ }}mass}}$

Complete answer:
The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol)
In the first question, we are given the atomic masses of Ne and Ca. The question tells you that a certain number of atoms are present in $4g$ of calcium, and we have to find how many grams of Ne will have those same number of atoms as in $4g$ of calcium. So first we will calculate the number of atoms in calcium.
For finding the number of atoms in calcium we use the formula
$Number{\text{ }}of{\text{ }}atoms{\text{ }} = {\text{ }}Na{\text{ }}\left( {avogadro's{\text{ }}number} \right){\text{ }} \times {\text{ }}\dfrac{{mass}}{{atomic{\text{ }}mass}}$
The value of Avogadro’s number is $6.022 \times {10^{23}}$, mass and atomic masses are given in the question
Thus, $Number{\text{ }}of{\text{ }}atoms{\text{ }} = {\text{ }}6.022 \times {10^{23}}{\text{ }} \times {\text{ }}\dfrac{{4g}}{{40g}}$
Hence number of atoms in calcium $= 6.022 \times {10^{22}}$
It is said that both have the same number of atoms. So the number of atoms in neon$= 6.022 \times {10^{22}}$. Now we will calculate moles of Ne from number atoms by using the formula
Thus number of moles of neon $= \dfrac{{6.022 \times {{10}^{22}}}}{{6.022 \times {{10}^{23}}}}$
$= 0.1moles$
Thus mass of Ne $= 0.1 \times 20$
$= 2g$
Therefore 2 grams of neon will have the same number of atoms as 4 grams of calcium.
For the second question, it is given that atomic mass of S$\; = 32u$
Thus molar mass of Sulphur $= 8 \times$atomic mass of Sulphur
$= 8 \times 32$
$= 256g$
Therefore molar mass of $S_8$ $= 256g$

Note:
Atomic mass has significance in solving the stoichiometric questions and in the mole concept of chemistry. Molar mass has unit $\dfrac{g}{{mol}}$. The value of Avogadro’s number is constant $6.022 \times {10^{23}}mo{l^{ - 1}}$. The molecular weight is equal to the mass of one mole of the substance.